1. Field of the Invention
The present invention relates to internal combustion engines in general, and more specifically, to engines equipped with heat recovery systems in order to have a higher thermal efficiency than conventional two, four, six, eight, or more stroke internal combustion engines.
2. Description of Prior Art
The internal combustion (IC) engine is well over 100 years old. The original US patent was issued to Nicolaus Otto on Aug. 14, 1877 for a four-stroke spark ignited (SI) engine. An earlier patent was issued to Otto for a four-stroke engine in Germany. A compression ignition (CI) engine, or Diesel engine, was invented a few years after this. Other than rotary engines, which were introduced many years later, internal combustion engines have powered our society and are found almost everywhere. These engines have primarily been both two and four stroke varieties.
Many improvements have been made to both the SI and CI engines over the years to improve their thermal efficiencies and to reduce their emissions of noxious chemicals, particularly unburned hydrocarbon (HC), carbon monoxide (CO), and mixed nitrous oxides (NOx). Thermal efficiencies have increased over this time frame from single digits up to about 32% in everyday usage. One research study showed a thermal efficiency approaching 43% on a four-stroke SI engine fueled with and optimized for neat methanol (Matthew Brusstar, et al., “High Efficiency and Low Emissions from a Port-Injected Engine with Neat Alcohol Fuels”, SAE Paper 2002-01-2743, 2002). This high efficiency was made possible by modifying the engine to make use of the higher octane of alcohol as compared to gasoline. The CI engines have achieved 52% to 57% thermal efficiency in large slow speed maritime applications. The MAN S80ME-C7 has a specific fuel consumption of 155.85 to 167.85 g/Kwh. These efficiencies are the peaks or maximums, not the average efficiencies. The ratio of weight to power output of IC engines has dropped over this same period, thus allowing their application in high power transportation demands all the way down to their use in hand tools and model airplanes.
An internal combustion (IC) engine always includes one or more cylinders. Within each cylinder is a reciprocating piston connected to a crankshaft, which converts the reciprocating motion of the piston to a circular motion. In some rare cases, the reciprocating motion can be used directly, such as in electrical power generation or in pile driving. Four strokes are performed in the conventional IC engine; these include air or oxidizer intake, compression, power or combustion, and exhaust. These form the complete cycle. Two stroke IC engines are also very common, but they are less efficient and emit more noxious chemicals than a four-stroke engine.
The main problem with internal combustion engines is the low thermal efficiency of the complete cycle. Thermal efficiency is defined as the useful work output of an engine divided by the fuel consumed at its lower heating value. The peak thermal efficiency for a SI engine may be 32% in an automotive application, but the thermal efficiency of the same engine at its normal operating point may be only 15 to 20% or lower. In the 20% thermal efficiency case, 80% of the heat energy in the fuel is discarded as waste heat and is not converted to useful work. In a conventional engine, this heat is lost through the exhaust and through the cooling system, whether the engine is air cooled or liquid cooled. Friction inside the engine also accounts for 10 to 25% of the gross work output from the engine. This friction ends up as heat exiting the engine, so this friction is already included in the peak efficiency figures given. There are ways of increasing the thermal efficiency of an engine, but in each case, there are trade offs. Increasing the compression ratio of an engine can raise engine efficiencies, but this is limited by the combustion characteristics of the fuel. Air-to-fuel ratios can also be varied. In this case, less than stoichiometric fuel can be utilized; meaning that excess air is present. Combustion in this case produced too much NOx in the exhaust. Combustion can also be greater than stoichiometric, thus producing an exhaust with unburned hydrocarbons (HC) and carbon monoxide (CO). The air-to-fuel ratio must be stoichiometric for the current after-treatment catalysts to give emissions which meet or exceed government mandates.
There are other ways of increasing the thermal efficiency of IC engines. These have included turbocharging, supercharging, recycling the heat, double or triple reduction of exhaust pressure, port fuel injection, direct fuel injection, homogeneous charge compression ignition, and other ignition regimes. Work is on going on variable valve timing, camless valve operation, and cylinder deactivation to name a few. These have increased and are increasing thermal efficiencies but these improvements need to occur at a more rapid rate. A paradigm shift in thermal efficiency is needed and is provided by this invention.
There have been many variations of the four-stroke IC engine in an effort to improve the thermal efficiency. Others have recognized this deficiency in the four-stroke engine and have made steps to recover and recycle this heat. These efforts have resulted in increasing the number of strokes from four to six or eight or more. In a six-stroke engine, strokes one through four generally include air intake, compression, power or combustion, and exhaust as in a conventional four-stroke engine. A fluid, usually water but sometimes air, is injected in stroke five to recover some of the heat remaining in the cylinder, piston, and head. This fluid is expanded or vaporized from the heat remaining in the metal of the cylinder, piston, and head and is thus pressurized without additional fuel being consumed. In the case of water being added, this water is vaporized to steam with its pressure dependent on the temperature and heat contained in the metal of the cylinder, piston, and head and is also dependent on the temperature and volume of water injected. This produces an additional power stroke without the introduction of additional fuel. Hot water is sometimes used rather than cold water and this allows the pressure developed during stroke five to be greater and thus to convert more heat to work. Stroke six is the exhaust stroke to remove either the vaporized water or heated fluid from the cylinder before repeating the air intake stroke. There are, of course, other variations of this theme but all give a second power stroke within the six strokes of the engine.
There are many examples of six stroke engines in the patent literature. Some of the earliest and most influential US patents include the following:                U.S. Pat. No. 1,217,788 by Liedtke teaches six, eight, ten, or more strokes with the second power stroke forward provided by steam from an external source. It teaches minimal heat recovery and no water jacket on the cylinder.        U.S. Pat. No. 1,324,133 by Still teaches a double let down of the combustion gas pressure.        U.S. Pat. No. 1,339,176 by Dyer teaches water injection in stroke five which is vaporized to produce a second power stroke. No heat recovery is taught as well as no water jacket to cool the cylinder.        U.S. Pat. No. 2,671,311 by Rohrbach teaches heat recovery, air preheat, and condensing the water exiting the cylinder.        U.S. Pat. No. 3,964,263 by Tibbs teaches enhancement of the surface area of the piston and head to promote vaporization of the injected water.        U.S. Pat. No. 4,143,518 by Kellogg-Smith teaches heat recovery and injection of hot water into the cylinder for an additional power stroke.        U.S. Pat. No. 4,402,182 by Miller teaches the introduction of water at the end of stroke 3 after detonation to provide more work. This is truly a modified four-stroke engine.        U.S. Pat. No. 4,433,548 by Hallstrom teaches the introduction of steam into stroke five from recovered heat. A steam condenser is also taught to increase the recovery of work.        U.S. Pat. No. 4,513,568 by Bajulaz teaches the use of heated compressed air to produce the second power stroke. Waste engine heat is used to heat the air.        U.S. Pat. No. 6,095,100 by Hughes teaches the recovery of heat from the exhaust by a water spray with the resulting steam used in a vane motor to generate more work.        Patent application 2007/360022977 by Bruce Crower teaches the injection of water in stroke five to produce a second power stroke. No heat recovery is taught. No water jacket is required to cool the cylinder.        
In an eight-stroke engine, strokes one through four generally include air intake, compression, power or combustion, and exhaust as in a conventional four-stroke engine. There are many variations after the first four strokes. Examples from the U.S. patent literature relating to eight-stroke engines include the following:                U.S. Pat. No. 1,217,788 by Liedtke teaches six, eight, ten, or more strokes with the second or greater power strokes provided by steam from an external source. It teaches minimal heat recovery and no water jacket on the cylinder.        U.S. Pat. No. 5,598,819 by Blackburn teaches a four-stroke engine which can go to an eight-stroke engine when the load requirement is reduced. There is only one power stroke for each four or eight strokes.        U.S. Pat. No. 5,732,677 by Baca teaches two air intakes for each power stroke and no heat recovery. This engine would run at a much higher compression ratio.        U.S. Pat. No. 6,363,907 by Arai et al teaches a control scheme for an engine running on four, eight, or 16 cycles.        U.S. Pat. No. 6,443,108 by Brehob et al teaches multiple less than stoichiometric ratio combustions before the last combustion which would be at a stoichiometric ratio. The combustion gases are run through multiple cylinders while adjusting the air-to-fuel ratio. Strokes include 4, 6, 8, or greater.        U.S. Pat. No. 6,918,458 by Hu teaches a double expansion of the combustion gases with no engine cooling and no heat recovery.        Patent application 2007/470044778 by Milovanovic et al teaches two or three fuel power strokes at different air-to-fuel ratios with different pressure let down cylinders.        Patent application 2007/540119407 by Hu teaches double reduction of exhaust gases in a ten-stroke rotary engine.        
The eight-stroke engine described in this invention has the first four strokes the same as other eight-stroke engines; however, strokes five through eight are different. The drawings and the descriptions that follow will clearly show those differences and the advantages of this eight-stroke engine over prior art.